The present invention relates to a method and a system for controlling a brake for motor vehicles, which can be electromechanically actuated by means of an actuator consisting of an electric motor and a drive mechanism positioned downstream of the electric motor.
The present invention relates to a method and a system for controlling a brake for motor vehicles, which can be electromechanically actuated by means of an actuator consisting of an electric motor and a drive mechanism positioned downstream of the electric motor.
A type of method or system for controlling an electromechanically actuated brake, for example, has been shown in the German published application DE 195 36 694 A1. The special characteristic of the system known in the prior art is that means are provided, which determine the position of an actuating element relative to the actuator, that a controller is provided which can switch between a first and a second control mode, with the actual position of the actuating element matching a set value in the first control mode and an actual signal representing a delay matching a desired signal in the second control mode, and that a decision circuit activates the first or the second control mode depending on a decision criterion.
In addition to the available space as well as a defined power input (basically the peak and continuous current should be as low as possible), above all the following requirements regarding the performance of the brake need to be taken into consideration when designing or dimensioning an electric motor suitable for the aforementioned electromechanically actuated brake (definition of the electric motor characteristic curve):
1) The distance between friction lining and brake disc (so called free travel) should be overcome as quickly as possible. Hence, relatively large distances have to be covered with a low level of force, which means that the electric motor has to run at as high a speed as possible.
2) Required clamping force and release gradients (high speed at low to medium torque).
3) Dosing the clamping force. For this purpose, relatively large forces and short distances are required, which means that the electric motor has to provide as great a torque as possible.
4) A required maximum clamping force. For the electric motor to be designed this means that it has to provide correspondingly high torque at low speeds.
5) Good reversing performance, which is required for traction control functions (e.g. anti-lock system, electronic driving stability control), also necessitates a correspondingly high motor torque.
These contrary requirements cannot at all or not optimally be fulfilled in the available space for the electric motor with the above-mentioned control known in the prior art
It is an object of the present invention to propose a method and a system for controlling an electromechanically actuated brake that allows the above-mentioned criteria to be fulfilled. For this purpose, the speed of the electric motor is to be increased under certain operating conditions, i.e. depending on demand, without reducing the available torque.
This object shall be solved in that the slope of the speed-torque characteristic curve of the electric motor is influenced corresponding to the desired actuating force as well as the actuating force gradient or the values representing these signals.
In order to realize the concept of the invention, the slope of the speed-torque characteristic curve of the electric motor is changed in such a way the weaknesses of those components of the electromagnetic field that a higher speed is reached at the same torque. Hence, the advantage that the electric motor can be designed with a higher torque constant, meaning that the peak current, e.g. when reversing, and the continuous current during stationary clamping operations is reduced.
For this purpose, a torque-producing current value and a field-weakening current value are calculated, which are transformed into the currents to be supplied to the electric motor.
The torque-producing current value preferably is calculated by evaluating the desired and actual or present actuating force, whereas the field-weakening current value is calculated by evaluating the actual state as well as a desired state of the brake.
According to another advantageous feature of the method, the evaluation of the actuating force is based on force control.
The field-weakening current value is calculated by means of a quality criterion, whose input values are actuating force, actuating force variation and values that determine the working point in the characteristic curve of the electric motor.
A first embodiment of the control system according to the present invention that can execute the above-mentioned method is characterized in that:
a) an actuating force controller is provided, to which the difference between a desired and an actual value of the actuating force is supplied as the input value, with said actuating force controller generating the set value of the torque-producing current on the basis of this;
b) a calculating module is provided, to which the desired and actual value of the actuating force, the set value of the actuating force gradient or a value representing the actuating force gradient, the actual value of the motor position as well as signals representing the actual value and set value of the motor speed are supplied as input values, whereupon said calculating module generates the set value of the field-weakening current on the basis of these input values;
c) with the set values of both the torque-producing and field-weakening current as well as the signal representing the actual value of the motor position being transformed into a signal in a current control, which represents the voltage to be applied to the electric motor.
In advantageous preferred aspects of the first embodiment of the control system according to the present invention, the actuating force gradient as well as signals representing the actual value of the motor speed are additionally supplied to the actuating force controller.
A second embodiment of the control system according to the present invention for executing the above-mentioned method is characterized in that
a) an actuating force controller is provided, to which the difference between a set value and an actual value of the actuating force is supplied as the input value, with said actuating force controller generating the set value of the motor speed on the basis of this;
b) the set value of the motor speed is compared to the actual value of the motor speed, and the difference reached thereby is supplied to a speed controller, which generates the set value of the torque-producing current therefrom;
c) a calculating module is provided, to which the desired and the actual value of the actuating force, the set value of the actuating force gradient, the actual value of the motor position as well as the signals representing the actual value of the motor speed are supplied as input values, with said calculating module generating the set value of the field-weakening current on the basis of these input values,
d) with the set values of the torque-producing current and the field-weakening current as well as the signal representing the actual value of the motor position being transformed into a signal in an electronic current control, which represents the voltage to be applied to the electric motor.
In a third embodiment of the control system according to the present invention for executing the above-mentioned method, it is proposed that
a) an actuating force controller be provided, to which the difference between a desired and an actual value of the actuating force is supplied as the input value with said actuating force controller generating a first set value of the motor speed on the basis of this;
b) a precontrol module be connected in parallel to the actuating force controller, and that the actual value of the actuating force as well as the set value of the actuating force gradient be supplied to such precontrol module as input values, with its output values corresponding to an additional set value of the motor speed as well as an additional set value of the torque-producing current;
c) the additional set value of the motor speed are added to the first set value of the motor speed and compared to the actual value of the motor speed, and that the difference arising therefrom is supplied to a speed controller, which generates the controlled set value of the torque-producing current on the basis of this;
d) the additional set value of the torque-producing current are added to said controlled set value, with the result of the addition being the set value of the torque-producing current;
e) a calculating module is provided, to which the desired and the actual value of the actuating force, the set value of the actuating force gradient, the actual value of the motor position as well as the signals representing the actual value of the motor speed are supplied as input values, with said calculating module generating the set value of the field-weakening current on the basis of the input values,
f) and the set values of the torque-producing current and field-weakening current as well as the signal representing the actual value of the motor position are transformed into a signal in an electronic current control, with said signal representing the voltage to be applied to the electric motor.
Further details, features and advantages of the invention are provided in the following description of an embodiment, with reference to the enclosed drawings. The drawings are as follows: